1. Field of the Invention
The present invention relates a method of manufacturing a bipolar transistor operated at a high speed.
2. Description of the Related Art
The base of a conventional self-aligned bipolar transistor is constituted by an internal base region, an external base region connected to a base electrode, and a link region for connecting the internal base region and the external base region to each other. In order to operate the bipolar transistor at a high speed, the link region is required to have a high impurity concentration, a predetermined small depth, and a low resistance.
In many cases, the link region is generally formed by an ion implantation method. Therefore, since ions implanted in a substrate are diffused by channeling, the depth of the link region cannot easily be set within a predetermined range.
In conventional technique, in order to set the depth of a link region within a predetermined range, the following methods are used. In the first method, a scattering layer for suppressing channeling and reducing the range of ions is formed in a substrate in advance. In the second method, the impurity concentration of a collector immediately below the link region is increased to suppress channeling, such that the effective implantation energy of the ions is decreased. In the third method, the surface of an internal base region is etched to relatively decrease the depth of the link region with respect to the internal base region.
These methods, however, have the following drawbacks. That is, in the first method, since the profile of the impurity of the link region plots a moderate curve from the surface portion of the substrate to the inner portion thereof, the link region obtained by locally forming a region having a high impurity concentration near the surface of the substrate cannot be formed. In the second method, since BF.sub.2 + is used as an impurity, the characteristics of a transistor are degraded by F (fluorine). In addition, in the first and second methods, radiation damage caused by ion implantation is not inevitably avoided. Especially, the radiation damage is larger in the second method than in a method wherein B (boron) is used as an impurity. In the third method, the substrate cannot be easily etched with high accuracy, and an emitter width is changed in accordance with the depth of this etching.